One of the steps of forming paper on a paper machine is the application of a coating material to the paper surface. The coating step may be an on line or an off line operation. During the coating step, there is a need to control the weight of coating material applied to the paper substrate in both a machine and a cross direction sense. Coat weight control in the cross direction sense requires the adjustment of force on a coating blade or a coating rod to control the weight of coating on a paper sheet. Precise coating application is necessary across the width of the sheet and thus there is the need to adjust coat weight variation by changing the force on the blade and hence the position of the blade for precise coating application. In the past this has been done by the application of a plurality of actuators along the length of the coating blade that press a backing bar up against the blade. The paper sheet passes between the blade and a backing roll. Variation of the force applied to the coating blade affects the coat weight across the width of the sheet.
It is normal practice to use a solid backing bar to give a smoothly varying force application on the coating blade in a cross direction sense. In some cases an air filled pressurized flexible tube is positioned between the backing bar and the coating blade. The backing bar must have sufficient rigidity to resist the dynamic forces caused by the coating material between the blade and the paper, and yet be sufficiently flexible to allow for adequate movement to effect the cross directional coat weight application. In addition the backing bar must be substantial enough to allow attachment of actuators in a robust manner without risk of detachment from the backing bar. One problem with a solid bar is that it requires high forces to cause the required deflection for cross directional coat weight control.
One approach taken to solve this problem is the use of a segmented backing bar where each segment is attached to an adjustment point so the segments are independent of each other. However, this approach causes certain problems, for example, it is difficult to maintain a clearance between the segments and the segments tend to jam together. Furthermore, it is difficult to get a smooth deflection and force profile applied to the coating blade in a cross directional sense over a number of adjacent actuator attachment points.
A flexible bar is needed to provide a system that can easily be bent with low force actuators and still provide a smoothly changing pressure application between actuators without the problems inherent in the segmented bar approach.